1. Field of Invention
The invention relates to a self-biasing active load circuit and a related high voltage power supply and, in particular, to a high voltage power supply configured to bias an optical element in a charged particle beam processing system.
2. Description of Related Art
Gas-cluster ion beams (GCIB's) are used for many applications, including etching, cleaning, smoothing, and forming thin films. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas clusters may consist of aggregates including a few to several thousand molecules, or more, that are loosely bound together. The gas clusters can be ionized by electron bombardment, which permits the gas clusters to be formed into directed beams of controllable energy. Such cluster ions each typically carry positive charges given by the product of the magnitude of the electron charge and an integer greater than or equal to one that represents the charge state of the cluster ion.
The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per individual molecule. The ion clusters disintegrate on impact with the substrate. Each individual molecule in a particular disintegrated ion cluster carries only a small fraction of the total cluster energy. Consequently, the impact effects of large ion clusters are substantial, but are limited to a very shallow surface region. This makes gas cluster ions effective for a variety of surface modification processes, but without the tendency to produce deeper sub-surface damage that is characteristic of conventional ion beam processing.
Conventional cluster ion sources produce cluster ions having a wide size distribution scaling with the number of molecules in each cluster that may reach several thousand molecules. Clusters of atoms can be formed by the condensation of individual gas atoms (or molecules) during the adiabatic expansion of high pressure gas from a nozzle into a vacuum. A skimmer with a small aperture strips divergent streams from the core of this expanding gas flow to produce a collimated beam of clusters. Neutral clusters of various sizes are produced and held together by weak inter-atomic forces known as Van der Waals forces. This method has been used to produce beams of clusters from a variety of gases, such as helium, neon, argon, krypton, xenon, nitrogen, oxygen, carbon dioxide, sulfur hexafluoride, nitric oxide, and nitrous oxide, and mixtures of these gases.
Typically, a GCIB processing system comprises one or more optical elements to extract the cluster ions from the ionizer, accelerate the extracted cluster ions to a desired energy, and focus the energetic, extracted cluster ions to define the GCIB. The kinetic energy of the cluster ions in the GCIB may range from about 1000 electron volts (1 keV) to several tens of keV. For example, the GCIB may be accelerated to 1 to 100 keV.
Therefore, by design, one or more optical elements operate at a high voltage, and generally float above the desired voltage due to the relatively high impedance of most high voltage power supply outputs. In order to shunt excess current, a resistor load is disposed between the terminals of the high voltage power supply. However, when varying the desired voltage across a range of possible operating voltages, the power dissipation in the resistor load can become excessive, particularly at high voltages since the power dissipation scales as the square of the voltage (i.e., P=V2/R, where P represents power dissipation, V represents voltage, and R represents resistance). This excessive power dissipation may be impractical at high voltages.